1. Field of the Invention
The present invention relates to a casting method for casting steel or the like material by means of a twin belt caster and also to a twin belt caster suitable for use in carrying out such a method.
2. Prior Art of the Invention
FIG. 1 shows a known twin belt caster. The twin belt caster has parallel trains of dam blocks 7 arranged to be disposed between opposing longitudinal edges of upper and lower belts 4 and 1. The caster is inclined downward so as to provide a casting angle .alpha. of 5.degree. to 15.degree. such that a free surface (referred to as "meniscus" hereafter) of melt 8 for allowing the melt 8 to be poured further is formed in the caster, as will be seen from FIG. 2.
As a result of the downward inclination, the upper belt 4 and the lower belt 1 are offset from each other in the direction of flow of the melt 8 by an amount which is represented by L in FIG. 1. In consequence, the melt 8 which is to be solidified to form a billet 9 starts to solidify at different timings at its portions adjacent to the upper belt 4 and the lower belt 1. Namely, the portions of the melt 8 contacting the upper and lower belts 4 and 1 start to solidify so as to form initial solidification shell which is rigid enough to withstand stress generated by shrinkage or contraction of the melt due to further solidification of the melt under this shell. The initial solidification zone adjacent to the upper belt 4 is shown to have a length l.sub.t, while the initial solidification zone adjacent to the lower belt 1 is shown to have a length l.sub.b. In consequence, an offset of a length l.sub.0 is formed between the terminal ends of the upper initial solidification zone and the lower initial solidification zone. FIGS. 3A to 3C show the melt 8 and the billet 9 in cross-sections taken along different planes which are represented by IIIA, IIIB and IIIC in FIG. 2. More specifically, in the cross-section taken along the plane IIIA, the melt 8 has been solidified only at the bottom contacting the lower belt 1 and both side walls contacting the walls of the dam blocks 7 so that a substantially U-shaped initial solidification shell has been formed, as shown in FIG. 3A. In the cross-section taken along the plane IIIB, the solidification has proceeded so that the lower portion 10 (see FIG. 2) of the U-shaped initial solidification shell has commenced to shrink in the breadthwise direction so as to provide a substantially inverse-trapezoidal cross-sectional shape of the initial solidification shell as shown in FIG. 3B. In the cross-section taken along the plane IIIC, the upper portion of the melt 8 has contacted the upper belt 4 so that an upper shell wall has been formed. In this state, the lower solidification shell has completed its solidification shrinkage and, therefore, has become rigid.
As the solidification further proceeds, the upper shell wall 11 also tends to contract. This tendency, however, is resisted by the rigidity of the lower shell wall 10 which has completed initial solidification. In consequence, a breadthwise internal stress is generated in the upper shell wall 11 so that the cracks c are formed in the upper surface o the billet 9 so as to extend in the longitudinal direction of the billet 9 as shown in FIG. 4, whereby the quality of the product billet is impaired undesirably. In addition, the product billet 9 exhibits an inverse trapezoidal cross-section due to difference in the amount of contraction between the upper solidification shell wall 11 and the lower solidification shell wall 10, thus degrading the quality of the product.